This invention relates generally to predicting the amount of noise in a gas turbine engine, and more particularly to a computer-implemented method that predicts the amount of broadband noise from the stator vane cascade in a gas turbine engine.
An aircraft gas turbine engine includes a fan section, a compression section, a combustion section, and a turbine section. An annular flow path for the working medium gases extends axially through the engine. The fan section has a rotor assembly that includes an array of rotor blades that are angled with respect to the approaching gas flow. The fan section also has a stator assembly that includes an array or cascade of stator vanes. Each vane is typically arranged in a radial direction outward from the center axis of the engine. The stator vane cascade is disposed axially downstream of the rotor blade array in the gas flow path. The vanes receive and guide the direction of the flow of gases exiting from the upstream rotor blade array. The stator vanes connect between an outer duct wall and an inner duct wall within the engine. The duct walls extend circumferentially with respect to the flow path to form the boundaries for the working medium gases in the fan section.
As the rotor assembly rotates, the blades do work on the gases to increase the pressure of the gases. The rotor blades also increase the velocity of the gases and direct the flow of gases from the engine axial direction to the blade rotation direction. The gases are then flowed past the rotor blade array to the stator vane cascade, which redirects the flow of gases to the axial direction. By reorienting the flow in this manner, the stator vane cascade increases the recovery of the flow energy of the gases into thrust.
As the working medium gases travel along the engine flow path, the gases are pressurized in the rotating fan and compression sections, which causes the gas temperature to rise. The hot, pressurized gases are burned with fuel in the combustion section to add energy to the gases. The gases are then expanded through the rotating turbine section to produce useful work for pressurizing the gases in the fan and compression sections. The expanding gases also produce thrust for propelling the aircraft forward.
The gas flow through the engine generates acoustic energy or noise. Aircraft engine manufacturers are concerned with the adverse effect of excessive noise levels on passengers, aircraft personnel, and residents in close proximity to airports. Due to increasingly stringent noise restrictions placed upon aircraft that operate in certain geographic areas and at certain times, there is a persistent need for quieter aircraft engines.
The principal sources of noise in an aircraft gas turbine engine are jet or exhaust noise, core noise, and fan noise. Jet noise results from mixing of the high-velocity engine exhaust gas stream with the ambient air. A considerable amount of turbulence is generated when these two streams, which are traveling at different velocities, mix together. This turbulence generates jet noise. In a turbofan engine, there are two exhaust streams; therefore, there are two sources of jet noise. One noise source is the turbulent mixing of the fan exhaust stream with the ambient air. The other noise source is the turbulent mixing of the engine core exhaust stream with the fan exhaust stream and the ambient air.
Core noise consists of compressor noise, combustion noise and turbine noise. Compressor and turbine noise are caused by the unsteady blade forces and fluid stresses when fluids are compressed for driving the turbines. Combustion noise results from the turbulence generated by the burning of fuel in the combustion chamber.
Fan noise is often the predominant noise source in a high-bypass ratio turbofan engine. Fan noise is caused by non-uniform gas flow exiting the rotor blades and impinging upon the stator vanes in the fan section. As each rotor blade passes through the gases, the blade leaves a wake or track of turbulent gases behind it at the trailing edge of the blade. This wake is commonly referred to as the rotor wake turbulent flow. Also, tracks of non-turbulent flow exist between the rotor blade array and stator vane cascade in the areas not directly behind the blades. The turbulent flow impinges on the stator vanes and generates much of the noise radiated by the engine. This fan noise is also commonly referred to as rotor/stator interaction noise.
In addition, secondary flow patterns exist adjacent the tips of the rotor blades due to the interaction of the blade tip with the boundary layer of the engine outer duct wall. This interaction introduces further turbulence (i.e., xe2x80x9ctip vortexxe2x80x9d) into the wake turbulent flow at the blade tip region. Also, turbulent flow emanates from the hub or root portion of the rotor blade (i.e., xe2x80x9chub vortexxe2x80x9d). The wake turbulent flow (i.e., xe2x80x9cdownwashxe2x80x9d) from rotor blades sweeping past stator vanes produces pressure fluctuations on the vane surfaces. These fluctuating aerodynamic pressures on the vane surfaces produce forces that generate noise.
The rotor blade wake turbulent flow has a steady component and a random component. The steady component is also commonly referred to as the harmonic or periodic wake component.
The random component is also commonly referred to as the broadband wake component. The random component is represented by the turbulent kinetic energy, which varies dramatically across the span of the stator vane inlet. The turbulent kinetic energy is often greater at the root or hub and tip regions of the rotor blade as opposed to the mid-span region of the rotor blade. The turbulent kinetic energy present in the blade root region is typically attenuated as it is absorbed downstream in the low-pressure compressor section of the engine.
The harmonic and broadband components of the wake turbulent flow are related to the noise spectrum components. The harmonic wake component causes harmonic or tonal noise and the broadband wake component causes broadband noise. Tonal noise is noise at specific frequencies, which are multiples of the rotor blade passage frequency. This tonal noise has a distinct sound that can be heard above the background noises. The amount of tonal noise generally depends upon the number of stator vanes and rotor blades, the geometry of the duct walls bounding the flow path for the working medium gases, the velocity of the gases, and the rotor speed. On the other hand, broadband noise is distributed over a wide range of frequencies, rather than being at specific, discrete frequencies. The noise signatures of modem turbofan engines tend to be dominated by broadband noise.
It is difficult to suppress or attenuate fan noise because of the interdependence of the mechanisms that contribute to this noise and the basic aerodynamic operation of the fan section of the engine. The prior art contains noise suppression structures adapted specifically for retrofit or original fit on an aircraft gas turbine engine. Typically, the noise suppression structure consists of sound attenuating liners applied to the nose cowl, the nose dome and the fan flow path components of the engine. In typical constructions, the sound absorption material lines the inlet duct and nozzle of a turbojet or turbofan engine to suppress the noise generated within the flow path. However, significant aerodynamic losses (e.g., thrust) result from the addition of noise suppression structures that provide for acceptable levels of tonal and broadband noise.
Another known method of attempting to reduce the fan noise involves selecting blade/vane ratios to satisfy the cut off criterion for propagation of noise at the fundamental rotor frequency. It is also known to increase the axial spacing between the rotor assembly and the stator assembly to reduce fan noise.
The aerospace community is aware of the potential for reducing the tonal noise component of the turbulent rotor wake by adjusting the angular physical positioning of the stator vanes in one or both of two different angular directions. This angular physical positioning is known as xe2x80x9cleanxe2x80x9d and xe2x80x9csweepxe2x80x9d. Vane sweep is the axial displacement of the vane such that the tip region of the vane is disposed farther downstream axially than the vane hub. Correspondingly, vane lean is a circumferential displacement of the vane stacking line relative to the radial direction of the vane. Lean of the vane tips is normally in the direction of blade rotation. Sweep and lean vane positioning may be performed independent of each other, or they may be done simultaneously. Swept and/or leaned stator vanes reduce tonal noise by reducing the severity of the rotor wake interaction with the vanes. The axial spacing (i.e., upstream, downstream) of the rotor assembly with respect to the stator assembly may also typically be varied with various sweep and/or lean configurations.
Recent empirical laboratory testing of an engine fan section has revealed the unexpected potential for use of stator vane sweep and/or lean physical positioning to reduce the broadband noise component of the rotor turbulent wake flow. See Woodward, R. P., et al., xe2x80x9cBenefits of Swept and Leaned Stators for Fan Noise Reductionxe2x80x9d, AIAA Paper 99-0479, presented at the 37th Aerospace Sciences Meeting and Exhibit, 1999, Reno, Nev., Jan. 11-14. Therein it was described that sweep only, and sweep together with lean, were both effective for reducing the amount of broadband noise at the stator vane inlet. The quantity of measured broadband noise was given as a function of various lean and sweep angles. The acoustic response results revealed significant reductions in both tonal noise and broadband noise beyond what could be achieved through the conventional approach of increasing the axial spacing between the rotor assembly and stator assembly.
Yet, instead of attempting to determine optimum vane sweep and/or lean positioning through the trial and error empirical testing approach, it is desired to accurately model various vane sweep and lean angles and the resulting acoustic response of the fan section. This is done in the present invention to accurately predict the amount of broadband noise for various sweep and/or lean angles. The model may be embodied in software that is rapidly executed in a computer implementation of the model. This way, an optimum arrangement in terms of fan noise reduction is quickly determined for the stator vane lean and sweep physical positioning in the fan section of a gas turbine engine.
An object of the present invention is to provide a computer-implemented method for accurately predicting the amount of broadband noise at the inlet of a stator vane cascade of a gas turbine engine.
According to the present invention, a model of a fan section of a gas turbine engine accounts for turbulence in the gas flow emanating from the rotor assembly and impinging upon the stator vane cascade at the inlet thereof. The model also allows for variations in the sweep and lean angles of the stator vanes. The model determines the resulting acoustic response of the fan section as a function of the turbulence and the lean and/or sweep angles of the vanes. The model may be embodied in software that is rapidly executed in a computer or workstation. The user enters desired values for the turbulence and the vane lean and/or sweep angles. The computer executes the program instructions embodying the model to solve the equations for the acoustic response of the stator vane. This way, an optimum arrangement in terms of fan noise reduction is rapidly determined for the stator vane lean and sweep physical positioning in the fan section of a gas turbine engine.
The present invention has a practical application and utility in that it provides for a computer-implemented tool to assist gas turbine engine designers in designing the fan section. The present invention allows the designer to xe2x80x9cvirtually designxe2x80x9d certain aspects of the stator vane cascade using a computer model, to achieve a minimum or optimum amount of fan noise emanating from the engine. Specifically, it allows the designer to vary the lean and/or sweep angles of the stator vanes, and then to assess the effect of those angles on the amount of fan noise generated by the interaction of the rotor wake turbulence with the stator vane cascade. This type of xe2x80x9cwhat ifxe2x80x9d analysis can be performed relatively much more accurately, rapidly and inexpensively in a virtual computing environment, as compared to a traditional laboratory empirical xe2x80x9ctrial and errorxe2x80x9d approach.
The above and other objects, advantages and features of the present invention will become more readily apparent when the following description of exemplary embodiments of the present invention is read in conjunction with the accompanying drawings.